The present invention relates to a pressure transducing assembly according to the preamble of claim 1.
It is known in the art that having measurement data in digital form offers the advantage over the well-known analogue representation of data to show very low additional signal noise and distortions in particular during the steps of further processing.
Picking-up measurement data from pressure measurements, for example in the infrasonic, ultrasonic or audible range, using conventional pressure transducing assemblies or digital microphones leads to signal distortions and noise impacts which arexe2x80x94in accordance with the tremendous low noise level capabilities of present data/sound playback devicesxe2x80x94in some cases disturbing.
Therefore, digital pressure pick-up devices/microphones have been developed with the aim to generate a digital equivalent of the analogue pressure-/sound signal at a very early stage of the signal processing chain.
In the known art digital pressure pick-up devices or microphones comprise an audio or pressure transducer on the basis of an analogue pressure/audio signal conversion process. These audio/pressure transducers of the conventional art contain at a first stage a transducer section which converts the mechanical analogue pressure value into an analogue electrical equivalent.
A microphone/pressure transducer generally produces a low-level electrical signal in response to that audible sound/pressure levels around the microphone/pressure transducer. This particular low-level electrical signal is then transmitted or conducted along an electrical pathway or cable to subsequent processing apparatusesxe2x80x94for example, a digital signal processing device such as an audio mixing and control sectionxe2x80x94where it is converted into a digital signal for further processing.
Along the particular electrical pathway the low-level electrical signal is affected by external noise and interference processes during transmission along the cable. Further, analogue amplificationxe2x80x94usually in the range of 40-60 dbxe2x80x94introduces noise and distortions.
Therefore, known digital microphones/pressure transducing assemblies utilize an analogue amplification stage immediately after the mechanical-to-electrical conversion of the sound/pressure so as to increase the signal-to-noise ratio.
Immediately after amplification an analogue-to-digital converter is connected which produces a digital equivalent to the analogue and amplified measuring signal.
Although known devices for modulating or converting analogue electrical signals do not produce further analogue noise per se, conventional electric negative feedback loops, for example in 1-bit analogue/digital converters, interact with further analogue equipment and in particular with difference/summation amplifiers which do introduce additional analogue noise to the sound/pressure signal in the feedback process, e.g. of a conventional delta-sigma modulator or transducer. Analogue amplification by itself is much worse than A/D conversion noise.
It is therefore an object of the present invention to improve the signal-to-noise ratio performance of known sound/pressure transducing assemblies.
This particular object is achieved with a pressure transducing assembly according to the generic part of claim 1 with the characterizing features of claim 1.
It is a further object of the present invention to reduce the parts count and assembly cost and size of sound/pressure transducing assemblies. Advantageous embodiments of the inventive transducing assembly are covered by the dependent claims.
State of the art pressure transducing assemblies for converting a received pressure into a digital pressure signal in general comprise pressure transmitting means, pressure receiving processing means and pressure signal processing means.
In accordance with the present invention the pressure transmitting means is adapted to receive a first or environmental mechanical pressure from an environment and to transmit said first pressure to said pressure receiving and processing means. Said pressure receiving and processing means is adapted to generate a second or internal mechanical pressure therein in accordance with said received first pressure and further to process said second pressure. Said pressure receiving and processing means comprises pressure signal generating means which is adapted to generate a first or analogue pressure signal being representative for said second pressure. Additionally, said pressure receiving and processing means has pressure compensating means to receive a pressure compensation signal and to generate an additional pressure within said pressure receiving and processing means in particular according to said analogue pressure compensation signal as to compensate said second pressure at least in part. The pressure signal processing means is adapted to receive and process said first analogue pressure signal. Furthermore, said pressure signal processing means has negative feedback capabilities to generate said analogue pressure compensation signal at least based on said received analogue pressure signal. Finally, said pressure signal processing means is adapted to generate a digital pressure signal having an integer number of bits and being representative at least for said internal pressure and/or said first or environmental pressure and to provide at least said digital pressure signal as an output signal.
A basic idea of the present invention is to exchange noise introducing analogue electronic elements of conventional sound/pressure transducing assemblies, in particular the analogue input amplifier together with the difference/summation-amplifier of negative-feedback capability, by means of mechanical components.
Therefore, the inventive pressure transducing assembly does not contain a mechanical-to-electrical signal transducer in connection with the amplifier and difference amplifier as an input stage. Instead, a received pressure signal from an environment is transmitted to pressure processing means, which indeed further processes the pressure, i. e. the physical or mechanical entity itself instead of its electrical equivalent as done by the conventional art.
The pressure processing means comprises a pressure signal generating means, which converts the pressure/soundxe2x80x94being processedxe2x80x94into an equivalent analogue electrical signal.
The equivalent electrical signal is then further processed in conventional manner by pressure signal processing means. Such a pressure signal processing means utilizes an integrator and a comparator to produce a digitized equivalent of the analogue electrical pressure signal. According to the negative-feedback capability of the pressure signal processing means the digital signal is fed back to the mechanical acting pressure receiving and processing means by using a digital-to-analogue converter which re-converts an integrated and digitized pressure signal into an analogue pressure compensation signal, the latter being impressed to the provided pressure compensating means of said pressure receiving and processing means. In particular 1-bitxe2x80x94i. e. on or offxe2x80x94or multi-bit digital-to-analogue conversion is applied.
The pressure compensating means then produces an additional pressure within said pressure receiving and processing means so as to compensate the pressure in said pressure receiving and processing means at least in part. Therefore, a mechanical realization of a negative-feedback control loop is realized by exchanging a conventional electrical amplifying and comparing stage by means of mechanical analogues. Therefore, no additional analogue electrical noise is introduced and accordingly, the output signal of the pressure signal processing means can have a better signal-to-noise ratio compared with the purely electrical or electronical realizations of sound/pressure transducing devices.
These conventional sound/pressure transducing devices are often known as delta-sigma transducers or modulators, and they are also called balanced charge transducers or modulators as they perform in part an electrical compensation of the analogue electrical input signal, thereby introducing additional noise on the electronic feedback signal.
Instead, the present invention therefore realizes a delta-sigma direct digital transducer which may be also called balanced pressure transducer or modulator, as it balances and compensates the pressure to be received and converted.
A preferred embodiment the inventive pressure transducing assembly comprises housing means into which at least said pressure transmitting means and said pressure receiving and processing means are assembled or embedded.
This ensures that the pressure transmitting means and the pressure receiving and processing means are fixed rigidly. Furthermore, the pressure receiving and processing means is protected against unwanted interactions, as pressure or material flow bypasses are avoided.
It is for instance possible to manufacture the inventive pressure/sound transducing assembly onto a single piece of a silicon chip, in particular using VLSI mikro-/nano-technology.
In the following the notation xe2x80x9cpressurexe2x80x9d is used. This notation is understood to include a pressure-distribution varying in time and space. Therefore, the notation xe2x80x9cpressurexe2x80x9d also includes xe2x80x9csoundxe2x80x9d being in the infrasonic, ultrasonic or audible range.
According to a further advantageous embodiment the pressure transmitting means of said inventive pressure transducing device has a first section being exposed to the environment and/or therefore to the first pressure to be received and converted. Furthermore, said pressure transmitting means has a second section being exposed to said pressure receiving and processing means. Therefore, the pressure transmitting means may be understood as a separating interface between the environment on the outside of said assembly and the pressure receiving and processing means inside the inventive assembly embedded in and protected by the housing means.
To protect said pressure receiving and processing means from being affected by pressure and/or material flow bypasses, said pressure transmitting means and/or said housing means are adapted and arranged so as to essentially isolate the pressure receiving and processing means from being directly affected by pressure and/or material flow from the environment. This ensures the avoidance of mechanical short circuiting.
The protecting effect is increased by having said housing means essentially mechanical rigid and/or impermeable to material exchange.
Furthermore, said pressure transmitting means is essentially impermeable to material exchange according to a preferred embodiment of the present invention.
Said pressure transmitting means may have at least a first membrane element with an environmental side face which is exposed to the environment and an inside face being exposed to said pressure receiving and processing means. Said membrane element is according to a preferred embodiment of the invention mechanical flexible so as to be capable to transmit the environmental pressure from said environmental side face of said membrane element to said internal side face. Therefore, said membrane element is arranged in said housing means so as to separate said pressure receiving and processing means from direct pressure and/or material flow from said environment.
The mechanical interaction and therefore the realization of a mechanical negative-feedback loop is obtained by having a cavity assembly arranged in said housing means as a part of said pressure receiving and processing means. Said pressure transmitting means is at least a part of a boundary of said cavity assembly against the environment.
According to another advantageous embodiment of the present inventive pressure transducing assembly said pressure signal generating means comprises at least a first separating element which is arranged to form an isolated detection compartment within said cavity assembly. Said detection compartment is isolated from said environment as well as from said transmitting means and has an outside face which is exposed to a remaining compartment of the cavity assembly, which itself has said pressure transmitting means as a part of its boundaryxe2x80x94and an opposed inside face which is exposed to the inside of the detection compartment.
According to that arrangement the cavity assembly is subdivided into a detection compartment which has as its only boundaries the housing and the pressure signal generating means. There is no direct connection to the remaining cavity assembly to the pressure transmitting means or to the environment.
On the other hand, there is formed another compartment within the cavity assembly being separated from the detection compartment which is called compensation compartment.
Therefore, the pressure compensating means comprises at least a second separating element which subdivides an isolated compensation compartment within said cavity assembly. Said compensation compartment is isolated from said environment as well as from said pressure transmitting means. It comprises an outside face that is exposed to a remaining compartment of the cavity assemblyxe2x80x94which itself contains a pressure transmitting means as a part of its boundaryxe2x80x94and comprises an opposed inside face being exposed to the inside of the compensation compartment.
Therefore, the compensation compartment has the same properties as the detection compartment and furthermore the detection compartment and the compensation compartment do not have an intersecting part. They do not have a common boundary. Said first and/or second separating element comprises at least a mechanical flexible membrane to allow for best mechanical interaction between the separating compartment of the cavity assembly.
In particular, a first and/or second membrane has at least in part an electrical conductive surface. Therefore, first and/or second membrane may act as a condenser or capacitor.
In the case that the first membrane is incorporated into said pressure signal generating means, according to the internal pressure of the pressure receiving and processing means the first membrane bends or vibrates so that the membrane""s shape is changed. This shape change leads to a change in the charge distribution, the electrical field distribution and/or the electrical voltage generated by said first membrane acting as a capacitor. The change of the electrical and/or mechanical properties of the first membrane may be detected and may serve as an analogue pressure signal for the internal pressure of the pressure receiving and processing means.
When said membrane incorporated into said pressure compensating means is electrical conductive and acts as a capacitor, the electrical charge distribution, electrical field and/or voltage on the capacitor may be altered in accordance with said pressure compensating signal. In accordance to the alteration of the electrical properties of the second membrane of the pressure compensating means, the membrane bends and alters its mechanical shape which leads to a change of the internal pressure of the pressure receiving and processing means in accordance with the compression or expansion of the medium inside.
Of course, the second membrane""s mechanical shape may be altered directly to produce a change in the internal pressure.
Said first and/or second membrane may contain at least in part electrostrictive and/or piezoactive material. It is also possible to use resistors embedded into said first sensing membrane. Mechanical stressers change the resistance of the embedded resistor and the change in the resistance may be measured and may serve as a measure for the pressure state of the cavity of pressure receiving and processing means. On the other hand a resistor may be also embedded in said second actuating or compensating membrane. By heating the resistor embedded in said second membranexe2x80x94for instance by applying an electrical current to said resistorxe2x80x94the membrane might expand and thus change its mechanical state. Therefore, said second membrane acts as an actuator.
Therefore, by changing the shape, i. e. curvature, effective surface or the like, said first membrane produces according to the electrostrictive/piezoactive properties of the material incorporated a change in the electrical state which can be detected directly as a measure for the pressure variation of the internal pressure of the pressure receiving and processing means.
On the other hand, changing the electrical properties of the said second membrane of the pressure compensating means leadsxe2x80x94also due to the electrostrictive/piezoactive properties of the material incorporatedxe2x80x94to a shape change of the second membrane and therefore to an alteration of the internal pressure in the pressure receiving and processing means.
According to a further preferred embodiment of the inventive assembly for transducing pressure said pressure signal generating means comprises sensor means being adapted to sense the electrical and/or mechanical state of said first membrane and to provide said analogue pressure signal being representative for said internal pressure of said pressure receiving and processing means.
Pressure compensating means may comprise probe and/or actuator means being adapted to change the electrical and/or mechanical state of the second membrane, respectively, according to said direct pressure compensating signal so as to force said second membrane to superpose said additional pressure to the remaining compartment of said cavity assembly.
According to a preferred embodiment of the inventive pressure transducing assembly pressure receiving and processing means and in particular pressure signal generating means and pressure compensating means comprise a common measuring/sensing and driving means, thus to simultaneously or successively measure/sense and drive the pressure within pressure receiving and processing means. Therefore, the known concept that a microphone is also a speaker is employed. A common measuring/sensing and driving means may be a membrane, a piezoelement or the like. Therfore, the sensing piezoelement or membrane and the pressure compensating piezoelement or membrane may be one and the same. Its action, i. e. the measuring/sensing process and the driving/compensating process, may be realized by organizing the element as a time multiplexed receiver and transmitter.
Said inventive assembly for transducing pressure may be adapted to receive and convert sound from the environment as a pressure varying with time and/or in space, in particular, in the audible, infrasonic and/or ultrasonic range.
In particular the inventive pressure transducing assembly may be adapted to receive and convert sound from the environment in the audible range from 15 Hz to 20 kHz. This ensures a proper application when using the pressure transducing assembly as a microphone. It may also be used as a tool for testing material quality or as an intrusion or motion detector.
The inventive pressure transducing assembly may be employed to a wide range of applications. Additional to applications in gaseous media such as air or the like the inventive assembling may be applied to measuring processes in liquids or the like, therefore, acting as an microphone or pressure transducer for liquids or fluids.
One can think of underwater microphones and of sonar applications. Furthermore, the spectral range of the pressure/sound signals to be measured by the inventive assembly may be chosen in a way that medical applications, for instance as ultrasonic devices or cameras, heartbeat monitors or the like are possible.
To further adjust for the proper application the cavity assembly and in particular the respective compartments may be filled with an appropriate gas or fluid, in particular with air. The medium must have appropriate properties with respect to compressibility, which is best fulfilled by gaseous media.
To avoid acoustical interferencesxe2x80x94as sound is equivalent to a pressure distribution changing in space and timexe2x80x94the inventive pressure transducing assembly may have a maximum linear dimension being smaller than half of the minimum wavelength xcexmin, which is defined by a respective dispersion relation
c(n,v)=xcex(n,v). v 
for v=vmax.
In this dispersion relation v denotes the frequency of the sound and has to be set to the maximum frequency vmax of the sound spectrum to be detected. n describes the material properties. xcex describes the wavelength, and c the propagation speed of the sound within said material.
In particular, in the application range of audible sound to the maximum frequency vmax=20 kHz, the inventive pressure transducing assembly and in particular the cavity assembly should have a maximum linear dimension of the cavity assembly which is small against 0,8 cm, when the cavity assembly is filled with dry air. For other applications the frequency range can be appropriately set and in particular the maximum frequency can be set to the far ultrasonic range, thereby reducing the maximum linear dimension of the cavity assembly according to the dispersion relation described above. For example for audio DVD and SACD devices the frequency range may cover 3 Hz to 48 kHz.
To ensure proper manufacturing in such low linear dimensions the inventive assembly for transducing pressure/sound may be manufactured by means of a micro-/nano manufacturing or engineering process as a micro-/nano-structure, in particular from a polymer solution or the like.
In such a manufacturing process a model of a preferred design of the three-dimensional structure of the pressure transducing device is subdivided into parts or slices of a distinct shape and width with a sequence of the parts or slices building in succession the complete design or model.
Each section or slice may then be projected by means of an optical arrangement to a distinct location within a polymer solution, with the projecting light leading the polymer solution in the focus of the projection to polymerize and therefore to build compact or solid material with the remaining parts of the solution not being accessed by focussed light remaining fluid.
Therefore, the design of the micro-structure of the inventive pressure transducing assembly can be built up slice by slice, including in particular the distinct membranes of pressure transmitting means, pressure signal generating means and pressure compensating means.
On the other hand micro-machining may be used to manufacture the inventive pressure transducing assembly on the basis of semiconductor substrates and in particular on the basis of silicon machining where the mechanical components and the electronics are built up and incorporated on a single chip preferably made of silicon.
The process may be realized by optical and/or x-ray lithography, chemical and/or physical deposition and/or etching.
These methods have been proven to be suitable for processing semiconductor material and in particular silicon.
It is further preferred that the inventive pressure transducing assembly is manufactured as a or on a piece of a silicon (Si), germanium (Ge), gallium arsenide (GaAs) or the like, in particular co-existing with electrical circuitry and further in particular using VLSI micro-/nano-technology. In reduced linear dimensions such a single-chip implementation can be used in particular in medical applications, for example in heart catheters for measuring the mechanical heart activity.